An ordinary electric angle grinder typically includes: a housing; a working mechanism disposed on a head portion of the housing and configured to grind or polish a workpiece; a motor disposed within the housing and configured to drive the working mechanism; a switch disposed within a handle portion of the housing and configured to activate or deactivate the motor; and a lever disposed right under the switch and configured to allow an operator to operate the switch in the housing through manipulating the lever. Once the motor is switched on, a grinding wheel of the working mechanism starts to rotate at a very high speed, during which, if the operator improperly operates the tool or is unfamiliar with the tool, the grinding wheel rotating at a very high speed might lead to body injury. For this reason, the grinder generally further includes a lock means disposed in proximity of the lever in order to prevent operation mistakes as much as possible.
Examples in this regard include a working tool as disclosed in PCT App. No. PCT/JP2009/065259, entitled “Working Tool”. In one embodiment, the working tool is an electric angle grinder including: a motor for driving a grinding wheel to grind or polish a workpiece; a motor housing for housing the motor, the motor housing having a portion defining a handle; a wheel guard coupled to a rear portion of the motor housing, at a location opposite to where the grinding wheel is mounted; a lever adapted to be operated by an operator by hand, the lever having a strip shape and being disposed above both the motor housing and wheel guard; and a switch disposed within the wheel guard and connected to the lever via an opening. The motor can be activated or deactivated by a manipulation to the lever by the operator.
In order to prevent operation mistakes, the electric angle grinder further includes a lock means disposed on the lever. The lock means includes a rotating shaft, an engagement portion and a manipulation portion. The principle of the lock means is that when the lock means is in a locked state, the engagement portion engages with a protrusion of the lever with the aid of an elastic force exerted by a spring, and the operator, thus, cannot operate the lever any more. To activate the motor to initiate the grinding or polishing operation of the grinding wheel, the operator needs to operate, with one hand, the manipulation portion with a force exceeding the elastic force of the spring so as to cause the lock means to rotate and the engagement portion to gradually detach from the protrusion of the lever. After the engagement portion becomes disengaged from the protrusion portion, the operator further needs to push the lever upwards, with the other hand, to switch on the switch. Moreover, in order to prevent a restoring force of the switch from causing the engagement portion of the lock means to re-engage with the protrusion of the lever and thus shutting down the motor, the operator further needs to always press the lever throughout the whole process of the grinding or polishing operation.
As indicated in the foregoing description, although this lock means facilitates mistaken operation prevention, it leads to disadvantages such as increase of the operator's physical workload, reduction of operational efficiency, and possible occurrence of undesirable motor shutdown.